1. Field of the Invention
The present invention is directed to a antenna for open MR devices having two spaced systems composed of planar conductors arranged on a carrier plate for currents that cross one another and that are capacitively shortened by tuning capacitors connected to ground at at least one end for tuning to the desired resonant frequency.
2. Description of the Prior Art
For imaging by nuclear magnetic resonance, a high-frequency, alternating magnetic field, whose frequency is dependent on the field strength of the basic field magnet, is required for exciting the spins. Typical frequencies are in the range from 8 MHz (0.2 T) through approximately 64 MHz (1.5 T).
So-called bird cage resonators are mainly used in cylindrical, i.e. closed systems. These, however, cannot be utilized in open systems such as, for example, C-shaped magnets since their conductor elements disturb the patient due to the desired openness, the openness being a desirable feature. Antennas have been specifically developed for this purpose that meet the demands of the open system, and are therefore more likely to be realized as planar structures.
Known antennas are composed, for example, of structures that a constructed like a micro-stripline that are capacitively shortened at one end or at two ends with capacitors and are thus tuned to the desired resonant frequency. In order to achieve an adequate field homogeneity, a number of such striplines are constructed next to one another and connected to one another. To construct a circularly polarized antenna, two of these arrangements that are rotated by 90xc2x0 relative to one another can be employed. Due to the arrangement above one another, a coupling between the two antennas occurs (given shortening at only one end) that must be compensated with suitable coupling elements such as, for example, capacitors or coils. When the two conductor arrangements are arranged on a common, double-sided carrier plate, then high capacitive currents through the printed circuit board with corresponding losses can occur due to the coupling.
An object of the present invention is to provide a circular, planar radio-frequency antenna for open MR devices that is constructed in a simple way and exhibits only low losses.
This object is inventively in a planar radio-frequency antenna for an open MR device wherein a planar metal layer is arranged on the carrier plate of each of the two antenna systems, one carrier plate being arranged above the patient and one carrier plate being arranged under the patient, with two currents being supplied respectively into the planar metal layer offset by 90xc2x0, and with tuning capacitors arranged at the feed point and at the opposite side. Only one such metal layer is present on each carrier plate. A single metal layer is present on each carrier plate.
The arrangement is preferably constructed such that two tuning capacitors on each side the feed point are provided.
The invention is based on the principle that two currents that flow exactly perpendicular relative to one another are ideally decoupled, and this is also true when the currents flow in the same plane. When the structure is completely symmetrical, then current can be supplied at two sides offset by 90xc2x0 relative to one another without having the systems influence one another. When, as usual, the currents are phase-offset by 90xc2x0 relative to one another, a circularly polarized magnetic field is generated as a result. Since an ideally symmetrical structure cannot be realized in practice, a certain, slight coupling of the systems nonetheless occurs, but this can be compensated in a very simple way by means of the tuning capacitors arranged at both sides of the feed points, since the capacitance can, so to speak, be shifted back and forth between these flanking tuning capacitors.
Particularly as an adaptation to the spherical homogeneity volume of the basic field magnet, it has proven expedient to employ a circular conductor arrangement, i.e. a circular metal layer, in addition to a quadratic metal layer. This provides an adaptation to the rest of the geometry since, of course, the gradient coil also is usually circular. The electrical advantage that is thereby gained is that the tuning capacitors in this arrangement can be uniformly arranged around the circumference of the metal layer. A steady change of the potential on the conductor is achieved as a result, thereby homogenizing the generated field.
In an embodiment of the invention, the antenna is optimized by connecting a low-loss dielectric material between the circular metal layer and the ground surface instead of the discrete capacitors. In this embodiment as well, tuning capacitors that flank the feed point of the currents are additionally needed.
In order to obtain an approximately uniform field having a diameter of, for example 40 cm with such an antenna, the diameter of the arrangement must lie on approximately the same order of magnitude. An increase of the B-field in the middle directly above the metal layer occurs compared to the B-field at the edge of the metal layer, which degrades the desired homogeneity. An improvement can be achieved in an embodiment of the invention wherein a central, circular recess is provided in the middle of the metal layer, the field being able to proceed therethrough. The field thus is attenuated in the middle region, so that a more uniform field course is achieved overall.